Radio receiver circuits



Jan. 27, 1948. J. D. HOLLAND ET AL 2,

' RADIO RECEIVER CIRCUIT Filed Jan. 12, 1944 F/GB.

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o 4 I i 1 a 1 -05 -2 4 l-e m I02 I05L Inventors 07 01/4916? s/ m/ m 0: M/mvo/ts John Douglas Haifa 114.

By DmmnDwe Win 017.

Patented Jan. 27, 1948 UNITED STATES PATENT OFFICE signments, to International" Standard Electric, Corporation, New, York, N. Y., a corporation of a Delaware Application January 12; 1944, Serial No. 517,928 In Great. Britain January 22, 1943 Claims (Cl 179-1715) The present invention relates to electrical signal transmission circuits, with particular reference to radio receivers,

When'the signals applied to a radio receiver are liable at times to rise to'an excessively high level, it is common practice to include amplitude limiting means in the receiver to prevent'it from being blocked by excessive overloading. A case in point arises in the communication arrangements between aircraft which may sometimes fly in close formation;

The limiting arrangements hitherto employed have not been completely satisfactory in. receivers in which automatic gain controlis used. It is the principal ob ect of-the present invention to provide an improved limiting arrangement-for such cases.-

According to :the invention, therefore, there is provided an electricsignal transmission system having at leastone thermionic valve "amplifying stage provided with automatic gain control arrangements. com rising means including a rectifier adapted to limit the amplitude of the signal voltage applied to the amplifying stage and so arranged that the automatic gain controlling voltage is'prevented from adversely affecting the action of the amplitude limiting means.

According to another aspect, the invention comprises an-electric' signal transmission -system having at least one-thermionic valve amplifying stage provided with automatic gain control arrangements, comprising means including a.rectifier adapted to limit the amplitude-ofthe signal voltage applied to the amplifying stage and'so arranged that the automatic gain controlling voltageoperates to augment-the action'of the amplitude limiting means.

Alternatively, the invention may be regarded as an electric signal transmission system having at least one thermionic valve amplifying stage provided with automatic gain control arrangements, comprising means including a rectifier adapted to limit the amplitude of" the signal voltage applied to 1 the amplifying stage; means for applying the -automatic::gain controlling-volt:- age tether-control grid'of the amplifying-valve andmeans for biassing the rectifier independently of the gain controlling voltage;-

The invention will lee-described with reference to the: accompanying drawing in which:

Fig. 1 shows-a schematic circuitdiagram of -a radio receiver-provided with amplitude limiting arrangements according to existing practice.

Fig.2 shows a diagram-which includes two 2 alternative forms of the limiting circuit according to the invention; and

Figs. 3 and 4 show performance curves of the arrangements of Figs. 1 and 2.

Fig; 1 has been included in order to show an example of an arrangement already in use, for the purpose of explaining the reasons for its imperfect operation. It shows the first amplifying valve V of a typical radio receiver having the control grid connected to the secondary winding of the aerial transformer T; The cathode is connected to earth through a biassing resistance R2 shunted by a bypass condenser C2. The anode is supplied from the high tension source connected to the terminal 4 through a parallel resonant circuit L104 tuned to the signal frequency. The aerial transformer is tuned by a condenser C1 connected across the secondary winding, the condenser C3-being a bypass condenser. The automatic gain control voltage derived at terminal 3 from somelater part of the circuit (not shown) is appliedto the control grid of V through a resistance R1 and the secondary winding of T. The amplified output from the valve V is applied to the remainder of the receiver circuit not shown) in any convenient way from terminals I and-2.

A diode D has its anode connected to the transformer T and its cathode to a suitable tapping point 'on the resistance R2 as indicated. which point may coincide with the cathode in certain cases. This diode isprovided forthe purpose of limiting the amplitude of the'signals applied to the control grid when this amplitude exceeds some predetermined value;

By the arrangement'described, a positive potential derived from the anode current potential drop in the-lower part of the resistance R; as far as'the tapping point, is applied to the cathode of the diode. This effectively biasses the'anode negatively to the-cathode and'the diode remains in a substantially non-conducting condition until the signalamplitude: exceeds the bias so applied. The presence of the automatic gain control voltage; however, interferes with the operation of the arrangement because this voltage is applied directly to-the anode of the diode. As the incoming signal level increases, so the automatic gain control voltage" becomes increasingly negative andthis increases the negative bias-of the anode of the diodesothat limiting doesnot occur until after the signal amplitude has exceeded some larger value than that desired; in other words the limitation occurs too late.

It isxtmbenoted that theactionof the automatic gain control voltage also decreases the anode current of the valve V when the signal level increases, and this will make the cathode of the diode D rather less positive relative to the anode which is in a direction to compensate to some extent for the direct action of the control voltage on the anode of the diode. compensation is however only partial.

The difficulty could of course be avoided by omitting the automatic gain control, but this is not a satisfactory solution, because it is generally necessary to compensate for variations in signal level for the weaker signals.

Fig. 2 shows the circuit of Fig. 1 modified according to the present invention, and includes two alternative arrangements. Those parts of the circuit which are the same as Fig. l are designated in the same way and will not be again described.

The control grid of the valve V is connected to the anode of the diode through a blocking condenser C5, and the automatic gain control voltage is applied through the resistance R1 to the control grid. It is therefore now removed from the anode of the diode. The secondary winding of the transformer T is connected directly to ground, the by-pass condenser C3 shown in Fig. 1 not now being required.

According to the first alternative, a biassing source of suitable voltage is connected to the terminals 5 and 6 so that the diode is biassed independently of the gain controlling voltage. The point at which limitation commences is now unaffected by the operation of the automatic gain control.

According to the second and preferred alternative advantage is taken of the variations of the anode current of the valve V to augment the amplitude limitation as the signal level increases. The biassing source connected to terminals 5 and 6 is omitted, and the cathode of the diode is connected instead to a tapping point on the resistance R2 as indicated by the dotted line in Fig. 2. The anode of the diode is now negatively biassed in the same way as described in connection with Fig. 1, and as explained, the negative bias decreases as the signal level increases, due to the effect of the automatic gain control on the anode current of the valve V. The stronger signals are therefore more severely limited, which is a desirable arrangement, and the advantage of the automatic gain control is at the same time secured for the weak signals.

The advantage obtained by the use of the invention in a typical case is indicated in Figs. 3 and 4, the values quoted being arbitrary. In Fig. 3 the voltage applied to the control grid is shown related to the voltage derived from the aerial for the following conditions:

Curve A.-Conditions of Fig. 1 but with the diode disconnected.

Curve B.Conditions of Fig. 1 with the diode connected.

Curve C'.Conditions of Fig. 2 (preferred alternative).

In Fig. 4 the curve shows the: relation between the input aerial voltage and the finally demodulated voltage at the output of the receiver, for the second alternative of Fig. 2, with the diode connected (curve D) and disconnected (curve E) respectively.

Fig. 3 indicates that while some improvement is obtained by the use of the diode in the con- Theamount of additional improvement. Fig. 4 shows that while an approximately linear relation between output and input is obtained for weak signals, when limitation begins, the preferred arrangement of the invention produces if anything a slight decrease in output level with increase in signal level, while if the diode is omitted, an abnormal increase in output level is obtained.

It will be understood that although in Fig. 2 the cathode of. the diode is biassed from a resistance connected in series with the valve V, it could equally well be biassed in a similar way from any other subsequent valve stage which is affected by the automatic gain control. Furthermore, the diode may be connected across the tuned circuit of any subsequent amplifying stage ventional arrangement, the preferred alternative instead of across the aerial transformer as shown in Fig. 2. For example, it could be connected across L1, the cathode being biassed from the next following stage.

t It should be pointed out that although the invention has been explained with reference to a .radio receiver, the principles are applicable to any signal transmission system having automatic gain control where amplitude limitation is required to deal with abnormal signal levels. Substantially the same results can be obtained by using some other type of rectifier instead of the diode, such as a copper oxide or selenium rectifier, provided that the frequencies involved are not so high that the self capacity of such rectifiers has a detrimental efiect.

What is claimed is:

1. An electric signal transmission system having at least one thermionic valve amplifying stage provided with means for securing automatic gain control and amplitude limiting means, said last means including a rectifier so connected as to limit the amplitude of the signal voltage applied to said amplifying stage, means connected so as to apply the voltage derived from said automatic gain means to the control grid of said amplitying valve, and means for biassing said rectifier independently of said gain controlling voltage.

2. System according to claim 1, also including a blocking condenser through which said rectifier is connected to said control grid of said amplifying valve, the automatic gain controlling voltage being applied to said control grid via a metallic conducting path, whereby said blocking condenser permits said rectifier to be independently biased.

3. An electric signal transmission system having at least one thermionic valve amplifying stage provided with means for securing automatic gain control and amplitude limiting means, said last means including a rectifier so connected as to limit the amplitude of the signal voltage applied to said amplifying stage, means connected so as to apply the automatic gain controlling voltage to the control grid of said amplifying valve, and means for biassing said rectifier independently of said gain con-trolling voltage, also includin a parallel resonant circuit tuned to the signal frequency, a source of biassing potential independent of said automatic gain controlling voltage and connections placing said resonant circuit, said source and said rectifier all in series with one another.

4. System according to claim 3, in which said biassing potential is derived from the anode current of said amplifying valve, whereby said potential varies according to the value of said anode current.

1:15.. System according to claim 3, in which the biassing means comprises a resistance connected in series with the cathode of said valve, the biassing voltage being derived from a predetermined intermediate point on said resistance.

6. System according to claim 3, in which said rectifier is a diode, and also including a blocking condenser connected between the anode of said diode and the control grid of said amplifying valve.

7. An electric signal transmission system comprising at least one thermionic valve having a control grid, an amplitude limiting rectifying means connected with the control grid, means biasing said rectifying means so as to limit the amplitude of the signal Voltage applied to the control grid, an automatic gain control means connected to the control grid of the valve, and a blocking condenser in the connection to the control grid between the connection for the rectifying means and that for the automatic gain control means.

8. An electric signal transmission system comprising at least one thermionic valve having a control grid, anode and cathode, an amplitude limiting rectifying means connected in shunt with the control grid, means biasing said rectifying means so as to limit the amplitude of the signal voltage applied to the input electrode including a resistance in the cathode circuit of the thermionic valve and a connection from the resistance to the rectifying means, an automatic gain control means connected to the control grid of the valve, and a blocking condenser in the connection to the control grid between the connection for the rectifying means and the automatic gain control means.

9. An electric signal transmission system hav ing at least one thermionic valve amplifying stage having a control grid, means connected with the amplifying stage for securing automatic gain control, and amplitude limiting means, said last means including a rectifier connected with the control grid to limit the amplitude of the signal voltage applied to said amplifying stage, and additional means connected in the circuit isolating said automatic gain controlling means from the amplitude limiting means whereby the voltage derived from the gain control means is substantially prevented from adversely affecting the action of said amplitude limiting means.

10. An electric signal transmission system having at least one thermionic valve amplifying stage having a control grid, means connected with the amplifying stage for providing automatic gain control, and amplitude limiting means, said last means including a rectifier connected with the control grid to limit the amplitude of the signal voltage applied to said amplifying stage, additional means connected in the circuit isolating said automatic gain controlling means from the amplitude limiting means whereby the voltage derived from the gain control means is substantially prevented from adversely affecting the action of the amplitude limiting means, and means in the amplifying stage connected with the amplitude limiting means whereby the voltage from said automatic gain control augments the action of the former.

JOHN DOUGLAS HOLLAND. DUNCAN DOVE ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,298,083 Fyler Oct. 6, 1942 2,299,391 Holmes Oct. 20, 1942 1,959,062 Place May 15, 1934 2,144,995 Pulvari-Pulvermacher Jan. 24, 1939 

